Remove references to ServiceContext

[aulorbe]

examples/learn/generation/llama-index/llama-index-intro.ipynb

Line 441 in 580de38

"index = GPTVectorStoreIndex.from_documents(\n",

LlamaIndex has deprecated ServiceContext, so let's remove that so our examples are up to date. https://blog.llamaindex.ai/llamaindex-v0-10-838e735948f8

[aulorbe]

Not sure when/if I'll get to this, but flagging for @jamescalam , @rschwabco , @beardouglas